git: Fix race condition when [un]staging hunks in quick succession (#26422)

- [x] Fix `[un]stage` hunk operations cancelling pending ones
  - [x] Add test
- [ ] bugs I stumbled upon (try to repro again before merging)
  - [x] holding `git::StageAndNext` skips hunks randomly 
    - [x] Add test
  - [x] restoring a file keeps it in the git panel
- [x] Double clicking on `toggle staged` fast makes Zed disagree with
`git` CLI
- [x] checkbox shows ✔️ (fully staged) after a single
stage

Release Notes:

- N/A

---------

Co-authored-by: Cole <cole@zed.dev>
Co-authored-by: Max <max@zed.dev>

crates/sum_tree/src/cursor.rs

@@ -406,6 +406,7 @@ where
         self.seek_internal(pos, bias, &mut (), cx)
     }
 
+    /// Advances the cursor and returns traversed items as a tree.
     #[track_caller]
     pub fn slice<Target>(
         &mut self,

crates/sum_tree/src/sum_tree.rs

@@ -225,6 +225,15 @@ impl<T: Item> SumTree<T> {
         }))
     }
 
+    /// Useful in cases where the item type has a non-trivial context type, but the zero value of the summary type doesn't depend on that context.
+    pub fn from_summary(summary: T::Summary) -> Self {
+        SumTree(Arc::new(Node::Leaf {
+            summary,
+            items: ArrayVec::new(),
+            item_summaries: ArrayVec::new(),
+        }))
+    }
+
     pub fn from_item(item: T, cx: &<T::Summary as Summary>::Context) -> Self {
         let mut tree = Self::new(cx);
         tree.push(item, cx);